Drilling fluids, commonly referred to as drilling muds, are complex mixtures of chemicals. They are required to cool and lubricate the drill bit, suspend formation cuttings, lift them to the surface, and control formation pressure during the drilling of oil wells.
For the most part, the liquid bases are aqueous solutions, oils or emulsions of aqueous and oily materials, to which viscosifiers, dispersants, emulsifiers, weighting agents, fluid loss agents, pH control agents, salts, lubricants, select polymers, corrosion inhibitors, biocides, are usually added to enable the muds to meet the needs for particular drilling operations.
Drilling muds are usually classified as either water-based muds (WBM) or oil-based muds (OBM), depending upon the character of the continuous phase of the mud, although water-based muds may contain oil and oil-based muds may contain water.
Oil-based muds generally use hydrocarbon oil as the main liquid component with other materials such as clays or colloidal asphalts added to provide the desired viscosity together with emulsifiers, polymers and other additives including weighting agents. Water may also be present, but in an amount not usually greater than 50 volume percent of the entire composition. If more than about 5% volume water is present, the mud is often referred to as an invert emulsion, i.e., water-in-oil emulsion.
Water based muds conventionally comprise viscosifiers, fluid loss control agents, weighting agents and other additives including lubricants, emulsifiers, corrosion inhibitors, salts and pH control agents. The water makes up the continuous phase of the mud and is usually present in any amount of at least 50 volume percent of the entire composition. Oil is also usually present in minor amounts but will typically not exceed the amount of the water so that the mud will retain its character as a water continuous phase material.
Potassium-muds are the most widely accepted water mud system for drilling water-sensitive shales. K+ ions attach to clay surfaces and lend stability to shale exposed to drilling fluids by the bit. The ions also help hold the cuttings together, minimising dispersion into finer particles. Potassium chloride (KCl) is the most widely used potassium source. Others are potassium acetate, potassium carbonate, potassium lignite, potassium hydroxide and potassium salt of partial hydrolyzed polyacrylamide (PHPA). For rheology control, different types of polymers are used, for example XC polymer (Xanthan gum) and PHPA. For fluid-loss control, mixtures of starch and polyanionic cellulose are often used. Carboxymethyl starch (CM starch), hydroxy propyl starch (HP starch), carboxymethylcellulose and sodium polyacrylate (SPA) are also used. PHPA is widely used for shale encapsulation.
Salt-water muds contain varying amounts of dissolved sodium chloride (NaCl) as a major component. Undissolved salt may also be present in saturated salt muds to increase density or to act as a bridging agent over permeable zones. Starch and starch derivatives for fluid-loss control and Xanthan gum for hole-cleaning are among the few highly effective additives for salt-water muds.
Sea-water mud is a water based mud designed for offshore drilling whose make-up water is taken from the ocean. Sea-water has relatively low salinity, containing about 3 to 4% by weight of NaCl, but has a high hardness because of Mg+2 and Ca+2 ions. Hardness is removed from sea water by adding NaOH, which precipitates Mg+2 as Mg(OH)2 and by adding Na2CO3, which removes Ca+2 as CaCO3. Mud additives are the same as those used in freshwater muds: bentonite clay, lignosulfonate, lignite, carboxymethylcellulose or polyanionic cellulose and caustic soda. XC polymer may also be used in place of bentonite.
Silicate-mud is a type of shale-inhibitive water mud that contains sodium or potassium silicate as the inhibitive component. High pH is a necessary characteristic of silicate muds to control the amount and type of polysilicates that are formed. Mud pH is controlled by addition of NaOH (or KOH) and the appropriate silicate solution. Silicate anions and colloidal silica gel combine to stabilize the wellbore by sealing microfractures, forming a silica layer on shales and possibly acting as an osmotic membrane, which can produce in-gauge holes through troublesome shale sections that otherwise might require an oil mud.
Lime-mud is a type of water based mud that is saturated with line, Ca(OH)2, and has excess, undissolved lime solids maintained in reserve. Fluid-loss additives include starch, HP-starch, carboxymethylcellulose (CMC) or polyanionic cellulose (PAC).
Apart from cooling and lubrification of the drilling bit, evacuation of the cuttings to the surface and control of the formation pressure, drillings muds must ensure to minimize invasion into permeable zones.
The drilling fluid creates a filter cake that imparts low permeability to the face of the permeable formation. The ideal filter cake comprises a relatively thin and hard layer as opposed to thick viscous coating. Pressure in the bore hole exceeds the pressure in the permeable formation and thereby creates the filter cake which further results in liquid from the drilling fluid moving into the permeable formation. This leaves a layer of the filter cake on the face of the hole. Liquid permeating this filter cake and the formation is called filtrate. As the thickness of the filter cake increases, the volume of fluid loss also increases. The thinner the filter cake, the lower the fluid loss. A thick wall cake will lead not only to high fluid loss, but also to a reduction in the diameter of the well bore.
The function of the fluid loss control agents is to delay, prevent or at least limit as far as possible fluid losses that may be sustained by the drilling fluids during the drilling operation. However, most of fluid loss control agents used in water based muds such as polyanionic cellulose, carboxymethylcellulose, starch, etc. give also rheology to the mud. This is therefore limiting the level of fluid loss control agent that can be used. In this context, an additive that would only control the fluid loss properties would allow more flexibility in term of level usage. This can become really critical when severe drilling conditions require an ultra-low permeability barrier between the well bore and the formation.
Additionally, the compounds added to the mud must withstand the high temperature/high pressure (HTHP) in the wells. Materials that are described in the art as HTHP fluid loss control aids have actually a poor stability under these extreme conditions of temperature and pressure.
For high salinity and high temperature conditions, acrylamido-methyl-propane sulfonate polymers (AMPS polymers) have been developed. However, these polymers are showing limited performance above 120° C.